Darwin, Incest and Plant Sex

17 February, 2009

In the heart of Berlin in the 1790’s a theologian named Spengel was hard at work investigating the sex of plants. Up until that time it was generally assumed that many plants, having both male and female parts, would fertilise themselves. Spengel spent much time examining plants and came to rather different conclusions that he published in his opus Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen. It went down like a lead balloon. No one was interested and his work languished unheeded and Spengel moved on from botany. If he was remembered at all it was by his fellow professors who would rather not talk about plant sex at all thank you very much.

Half a century later Charles Darwin was worried about incest and inbreeding. For his theory of Natural Selection to work there had to be enough variety within a population. Sex provided this variety but what if a population kept breeding within a small group? This was more than just idle curiosity for Darwin, he had married his first cousin and indeed his family and that of his wife had been interbreeding for many generations. It was well known amongst livestock breeders that closely related breeding pairs gave rise to poor offspring. Three of Darwin’s ten children died in childhood – he wondered if his close relationship to his wife had left him with weaker children. With these thoughts on his mind he did what he excelled at – doing an insane amount of experiments.

Darwin performed thousands of experiments in search for evidence for his theories. No armchair philosopher, Darwin was the epitome of the experimental scientist. He turned his attention to flowers, which he felt needed an explanation to fit with Natural Selection – why would plants have created these beautiful and complex structures unless there was a reason for them? It was known that insects sometimes carried pollen between plants but this was seen as a rare occurrence. Darwin set out to show that this was the main way in which the flowering plants reproduced and, inspired by Spengel’s work, he devised some simple experiments to demonstrate what would happen if flowers self fertilised as was thought to be the case.

He created two groups of plants. One group was forced to self fertilise. This is a rather easy thing to do – essentially a bag is placed over the flower so that the only interaction can come from the same plant – if fertilisation occurs it has to have been from the same plant. For the other group he castrated the flowers – pulling out the stamen before the plant was fully fertile and thus any fertilisation would have to come from another plant. After a few generations the results were clear. There were fewer self-fertilised plants, and those that had survived were stunted compared to the cross-fertilised crop. This had to mean that the main strategy for reproduction for those plants was cross-pollination, which meant the insects were pivotal in the process. And this gave an answer to why the plants had flowers – they were there to attract insects.

Darwin’s experiments and results (published in his book On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing ) convinced the world in a way that Spengel hadn’t. Perhaps it was the snappy title, or that his experiments were more convincing than the theologians. In any case, plant sex had finally come into its own and took its place as part of the scientific knowledge of the age.